The objective of this PPG application is to develop a rational approach to therapy of sudden cardiac death through understanding of the pathogenesis of ventricular fibrillation (VF) at the mechanistic level. Previous collaboration between these investigators has led to the recognition that membrane voltage-driven dynamic factors, such as electrical restitution and excitability, contribute to initiation and maintenance of wavebreak during VF. We demonstrated two types of VF. Type 1 VF is associated with steep action potential duration (APD) restitution, normal excitability and multiple wavelets. Type 2 VF is associated with flat APD restitution, reduced excitability and spatiotemporal periodicity. We also developed methods to simultaneous map membrane potential (V) and intracellular calcium (Cai) in normal and diseased rabbit ventricles, and to detect spatial and electromechanical APD and Cai transient alternans. Preliminary results suggest that, along with voltage-driven dynamic factors, Cai cycling is a critical factor that controls multiple aspects of dynamic wave stability, including spatial alternans, ventricular tachycardia (VT) to VF transition, and the generation of new wavebreaks during VF. We also demonstrated that the spatial alternans and APD heterogeneity are significantly influenced by underlying structural heterogeneity, for example, by the insertion of papillary muscle into the left ventricular free wall. Consistent with the central theme of the PPG, to study the interaction between V-Caj dynamics and electrical/anatomical tissue heterogeneity in VF, we will use simultaneous optical voltage and Cai mapping and micoelectrode transmembrane potential recordings in intact rabbit ventricles to determine the relationship between V-Cai cycling dynamics, spatial alternans and wavebreak. We will investigate the role of elevated Cai and spontaneous Ca release in creating discordant alternans and wavebreaks initiating and maintaining VF. Specific Aim 1 will concentrate on normal rabbit ventricles. Specific Aims 2 and 3 will focus on diseased rabbit ventricles with increased electroanatomical tissue heterogeneity, due to non-ischemic cardiomyopathy and ischemic cardiomyopathy, respectively. Data analysis and interpretation will be facilitated interactively with theoretical studies in Projects 1,2 and 4. We expect that these studies will improve the understanding of the mechanisms of ventricular fibrillation and lead to better management of patients at risk of sudden cardiac death.